In recent years, elimination of solvents and use of water-based constitution have been demanded in the field of recording materials and an inking material utilized for printers, printing machines, markers and writing tools. Especially, there have been widely used recording materials comprising mainly an aqueous solution of a water-soluble dye and those comprising mainly a fine particle dispersion of a pigment for a water-based ink of ink-jet recording.
In a water-based ink comprising a water-soluble dye, there is used an aqueous solution of a water-soluble dye selected from the group of dyes that are categorized mainly into acid dyes, direct dyes and some food dyes. To the aqueous solution is added a glycol or an alkanolamine as a wetting agent, a surfactant or an alcohol to adjust the surface tension, thickeners and other agents according to need. The water-based inks using water-soluble dyes are most commonly used because of their high reliability of anti-clogging at pen tips or in a printer. The water-based inks using water-soluble dyes, however, easily cause blotting on recording paper because they comprise an aqueous solution of dyes. Therefore, they have only limited uses and their print quality tends to be deteriorated due to blotting of the ink. In addition, water-soluble dyes that only permeate into the recording material and are then dried to adhere in the recording paper, can hardly be said to be “dyed”. Thus, the light fastness of such water-based inks is not fully satisfactory.
Further, proposals of coloring water-dispersible resins with an oil dye or a hydrophobic dye as an ink utilized for ink-jet recording have been made in order to solve problems of low water resistance and low light fastness of water-based inks utilizing water soluble dyes. Inks utilized for ink-jet recording using emulsion polymerization particles or dispersed polymerization particles colored with an oil dye have been proposed, for example, in JP-A No. 55-139471 (the term, JP-A refers to an unexamined and published Japanese Patent Application), JP-A No. 58-45272, JP-A No. 3-250069, JP-A No. 8-253720, JP-A No. 8-92513, JP-A No. 8-183920 and JP-A No. 2001-11347.
Proposals have been made of not only coloring water-dispersible resins with an oil dye or hydrophobic dye but also utilizing colored fine particles comprising a colorant and a resin that is coated with the colorant, and also utilizing colored fine particles comprising a colorant and a resin further coated with a film-forming resin on the surface of the particles.
In a water-based ink utilizing these colored fine particles, there are problems in that the effect to improve light fastness is decreased and enhancing properties such as dispersion stability and discharge stability, which are needed for an ink utilized for an ink-jet system, are difficult, when the dyes remain on the surfaces of the particles or the dyes are separated from the particles.
On the other hand, in a pigment ink comprising mainly fine particle dispersion of a pigment, disclosed have been several proposals of coating the surface of a pigment with a film-forming resin in order to reduce problems of not obtaining enough density or poor color reproduction quality due to bronzing, and further, to improve light fastness, dispersion stability and discharge stability. Examples of coating a pigment with a resin are described in JP-A No. 8-269374, JP-A No. 9-151342, JP-A No. 10-88045 and JP-A No. 10-292143.
The above-mentioned colored fine particles produced by covering colored particles or colorant fine particles such as pigment particles have the possibility of overcoming problems accompanied with a water-based ink so as to achieve excellent recording quality. However, there still remain problems of dispersion stability after preparation of an ink, and requires special techniques in the production process. Additionally, dispersion stability in an aqueous medium is not fully sufficient and as a result, stability as an ink and discharge stability of an ink do not reach sufficient levels as required.
Accordingly, an object of the present invention is to provide a colored fine particle dispersion used in an ink for an ink-jet system that can be manufactured stably and having excellent dispersion stability. A further object is to provide a water-based ink comprising the colored fine particle dispersion and utilized for an ink-jet system having good stability. A still further object is to provide an image forming method by using the ink.